[GreenKeys] Relay suggestions

[John Nagle]

> From: Eric<epvgk at limpoc.com>
> To: Jim Haynes<jhhaynes at earthlink.net>,Joe Herdler
> 	<joe.herdler at yahoo.com>
>>
> On June 22, 2019 1:30:22 PM PDT, Jim Haynes<jhhaynes at earthlink.net>  wrote:
>> Someone suggested an optocoupler, and probably a better choice is a
>> solid state relay with optical isolation.  Attached is a document on
>> the
>> subject by somebody.  In addition to the relay you'll want to put in
>> something like back-to-back Zener diodes or an R-C snubber to limit
>> the peak voltage on loop opening to something that won't blow the
>> relay.

There are lots of good solid state relays. The Omron G3VM-401B
is known to work with 60ma 120VDC teletypes. This can handle
up to 400 volts. Turn-on time 0.3ms. turn-off 0.1ms. So it's
fast enough for 45 baud.  240mA, so more than enough current
capacity.

Data sheet: 
https://omronfs.omron.com/en_US/ecb/products/pdf/en-g3vm_401b_e.pdf

These cost about $2.50. This part is a few years old, and you
can get better specs for about half that now.

I've used a 1000pf ceramic in series with 100 ohms across the selector
magnet as a snubber. That gets the spike down to well below
400V.  Diodes are not as suitable; they short the selector
magnet during turn-off, which means current flows longer
and drop-out time is increased.

			John Nagle